In general, an image forming apparatus such as a laser printer includes an optical scanner for exposing the surface of a photoconductor to light. According to one known type, an optical scanner includes a light source (e.g., semiconductor laser), a light deflector (e.g., rotary polygon mirror) configured to reflect and deflect a laser beam from the light source for rapidly sweeping the surface of the photoconductor with the laser beam, a plurality of scanning lenses (e.g., f-theta lenses) through which the laser beam having been deflected by the light deflector passes, a beam detector (e.g. an opto-electronic element) configured to receive the laser beam for determining an image writing timing, and a mirror for reflecting the laser beam having been reflected by the light deflector toward the beam detector. In this arrangement, the mirror is disposed facing one scanning lens.
However, this arrangement of the optical scanner is disadvantageous because a part of the laser beam (i.e., reflected light) having been reflected at the incident-surface or the emission-surface of the scanning lens may possibly be further reflected at an end surface of the mirror and directed into the scanning lens and finally reach the surface (i.e., a target surface to be scanned) of the photoconductor drum. The reflected light reaching the surface of the photoconductor drum may result in a ghost image which deteriorates the overall image quality.
In view of the above, it would be desirable to provide an optical scanner which can suppress the reflected light from entering a target surface to be scanned, so that the deterioration of the image quality can be suppressed.